1. Technical Field
The present invention relates to a sheet feeding apparatus, for example, in a sheet folding apparatus for folding a sheet with an image formed thereon, and more particularly, to improvements in the skew correction mechanism for correcting skew of the sheet front end to feed to a roller pair rotating to feed the sheet inside the apparatus.
2. Description of the Related Art
Generally, the skew correction mechanism of this type of sheet feeding apparatus is widely used as a paper handling mechanism in a printing press, printer apparatus, copier, etc. As the typical mechanism, such a mechanism (register roller mechanism) is known that a sheet is fed from the upstream side to a roller pair coming into press-contact with each other in a halt state, is curved in the shape of a loop, thereby aligning the front end, and then, is carried in the apparatus by rotating the roller pair. Further, another mechanism (gate stopper mechanism) is known in which a gate stopper is provided on the upstream side of a roller pair to strike a sheet against the stopper surface, the sheet is thereby curved in the shape of a loop, and the sheet front end is aligned.
The former register roller mechanism is comprised of a structure in which inside the apparatus a pair of rollers are disposed, a register area for curving the sheet is further disposed on the upstream side of the rollers, the sheet front end is struck against a nip point of the halted roller pair and regulated, and the sheet is curved inside the register area. By this means, the sheet front end is aligned in the nip point in the direction orthogonal to the transport direction. Thereafter, when the roller pair is rotated, the skewed sheet is corrected, and carried in the apparatus. In adopting such a mechanism, it is necessary to adopt a driving mechanism in which the roller pair is controlled to rotate at timing for feeding the sheet.
In the latter gate stopper mechanism, for example, as disclosed in Japanese Unexamined Utility Model Publication No. H03-035954 and Japanese Examined Utility Model Publication No. H08-005966, a stopper member for striking the sheet front end to regulate and a register area are formed on the upstream side of a roller pair (or transport means such as a belt) for carrying the sheet in the apparatus. Then, the stopper member is provided with stopper driving means for shifting the stopper member to positions between a position entering inside the path and another position retracted from the path. By this means, the front end of carried-in sheet is regulated by the stopper member in front of the rotating rollers, and the sheet is curved in the register area. Then, the front end of the skewed sheet is aligned, and concurrently with retracting of the stopper, the sheet enters the nip point of the rollers.
In other words, in Japanese Unexamined Utility Model Publication No. H03-035954 and Japanese Examined Utility Model Publication No. H08-005966, a pair of rollers are provided in the transport path, the gate stopper member is axially supported to be swingably in front of the rollers, and the stopper member is coupled to an operating solenoid. Then, on the upstream side of the gate stopper are disposed the register area and transport rollers for transporting the sheet toward the stopper.
As described above, the skew correction mechanism in which the gate stopper is disposed on the upstream side of a processing position inside the apparatus to feed the sheet to the processing position in a correct attitude has been widely known in Japanese Unexamined Utility Model Publication No. H03-035954, Japanese Examined Utility Model Publication No. H08-005966, etc. In such a skew correction mechanism, it is possible to rotate the roller pair for carrying a sheet in the processing position independently of paper feed timing, in relation to the driving mechanism or in order to execute another job. Concurrently with this merit, after the stopper releases the locked sheet on the upstream side to the rotating roller pair, the sheet front end moves to the nip point by its nerve.
In other words, FIG. 17 shows the conventional gate stopper mechanism, and a regulation surface 100x of a stopper 100 rotates on a swing shaft 101. At this point, positioning the regulation surface in a nip point N of a roller pair 103a, 103b results in an unstable state of the sheet front end that may be struck by the roller 103a side or roller 103b side, concurrently with constrains in space. Therefore, conventionally, the regulation surface 100x of the stopper 100 is disposed on the upstream side of the nip point of the rollers, and the sheet is shifted from the regulation surface to the nip point N by its elastic force (nerve).
At this point, conventionally, the regulation surface 100x of the stopper is shifted between the operating position and the retracted position in the shift trajectory in the direction orthogonal to the path. Therefore, in the sheet locked in the curved state by the regulation surface, since the sheet front end moves to the nip point N of the rollers by the elastic force (nerve) of the sheet itself concurrently with release by the stopper, the sheet strikes the roller 103a when the sheet front end is curled upward, while striking the roller 103b when the sheet front end is curled downward.
Accordingly, after the sheet is released from the regulation surface 100x, the sheet front end arrives at the nip point N of the rollers, or is displaced toward one of up and down rollers to strike, and thus becomes unstable. The unstable behavior of the sheet front end becomes a cause of sheet front end bending or newly causing a skew.